inhabit the roots of many trees in a beneficial association that aids the tree in extracting
nutrients from the s oil?
The plain truth is that this critically important subject of the ecology of the soil has been largely
neglected even by scientists and almost completely ignored by control men. Chemical control
of insects seems to have proceeded on the ass umption that the s oil could and would s us tain
any amount of ins ult via the introduction of pois ons without s triking back. The very nature of
the worl d of the s oil has been largely ignored. From the few s tudies that have been made, a
picture of the impact of pesticides on the soil is slowly emerging. It is not s urpris ing that the
studies are not always in agreement, for soil types vary so enormously that what causes
damage in one may be innoc uous in anothe r. Light s andy s oils s uffer far more heavily than
humus types. Combinati ons of chemicals seem to do more harm than separate applications.
Des pite the varying res ults , enough s olid evidence of harm is accumulating to cause
apprehe ns ion on the part of many s cientis ts. Under s ome conditions , the chemical conversions
and transformations that lie at the very heart of the living world are affected. Nitrification,
which makes atmospheric nitrogen available to plants, is an example. The herbicide 2,4-D
causes a temporary inte rrupti on of nitrification. In recent experi ments in Florida, lindane,
heptachlor, and BHC (benzene hexachloride) reduced nitrification after only two weeks in soil;
BHC and DDT had significantly detrimental effects a year after treatment. In othe r experime nts
BHC, aldrin, lindane, heptachlor, and DDD all prevented nitrogen-fixing bacteria from forming
the neces s ary root nodules on leguminous plants. A curious but be neficial relation between
fungi and the roots of higher plants is s erious ly dis rupted. Someti mes the problem is one of
ups etting that delicate balance of populations by which nature accomplishes far-reaching aims.
Explos ive increases in s ome kinds of s oil organis ms have occurred when others have been
reduce d by ins ecticides , dis turbing the relation of pre dator to prey. Such changes could easily
alter the metabolic activity of the soil and affect its productivity. They could als o mean that
potentially harmful organisms, formerly held in check, could escape from their natural controls
and ris e to pes t s tatus.
O ne of the mos t i mp ortant things to remember about insecticides in soil is their long
pers is tence, meas ured not in months but in years. Aldrin has been recovere d after four years ,
both as traces and more abundantly as converted to dieldrin. Enough toxaphene remains in
s andy soil ten years after its application to kill termites. Benzene hexachloride persists at least
eleven years; heptachlor or a more toxic derived chemical, at least nine. Chlordane has been
recove red twelve years after its application, in the amount of 15 per cent of the original
quantity.
Seemingly moderate applications of insecticides over a period of years may build up fantastic
quantities in soil. Since the chlorinated hydrocarbons are pers is tent and long-lasting, each
application is merely added to the quantity remaining from the previous one. The old legend
that ‘a pound of DDT to the acre is harmles s ’ means nothing if s praying is repeated. Potato soils
have been found to contain up to 15 pounds of DDT per acre, corn s oils up to 19. A cranberry
bog under study contained 3 4.5 pounds to the acre. Soils from a pple orchards s eem to reach
the peak of conta mination, with DDT accumulating at a rate that almost keeps pace with its
rate of annual application. Even in a single season, with orcha rds s prayed fou r o r more times,
DDT res idues may build up to peaks of 30 to 50 pounds. With repeated s praying over the years
the range betwee n trees is from 26 to 60 pounds to the acre; under trees , up to 113 pounds.
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